Sometimes dubbed the Golden Age of Aviation, the period in the history of aviation between the end of World War I (1918) and the beginning of World War II (1939) was characterised by a progressive change from the slow wood-and-fabric biplanes of World War I to fast, streamlined metal monoplanes, creating a revolution in both commercial and military aviation. By the outbreak of World War II in 1939 the biplane was all but obsolete. This revolution was made possible by the continuing development of lightweight aero engines of increasing power. The jet engine also began development during the 1930s but would not see operational use until later. During this period civil aviation became widespread and many daring and dramatic feats took place such as round-the-world flights, air races and barnstorming displays. Many commercial airlines were started during this period. Long-distance flights for the luxury traveller became possible for the first time; the early services used airships, but, after the Hindenburg disaster, airships fell out of use and the flying boat came to dominate. In military aviation, the fast all-metal monoplane equipped with retractable landing gear — first placed into production by the Soviet Union with the Polikarpov I-16 of 1934 — emerged in such classic designs as the German Messerschmitt Bf 109 and the British Supermarine Spitfire, which would go on to see service in the coming war. Airships A number of nations operated airships between the two world wars, including Britain, the United States, Germany, Italy, France, the Soviet Union and Japan. This period marked the great age of the airship. Before the First World War, pioneers such as the German Zeppelin company had begun passenger services, but the airships constructed in the years following were altogether larger and more famous. Large airships were also experimented with for military purposes, notably the American construction of two airborne aircraft carriers, but their large size made them vulnerable and the idea was dropped. This period also saw the introduction of non-flammable helium as a lifting gas by the United States, while the more dangerous hydrogen continued to be used since the United States had the only sources of the gas at that time, and would not export it. In 1919 the British airship R34 flew a double crossing of the Atlantic and in 1926 the Italian semi-rigid airship, Norge was the first aircraft confirmed to fly over the North Pole. The